compute ID group-ID ke
compute 1 all ke
Define a computation that calculates the translational kinetic energy of a group of particles.
The kinetic energy or each particle is computed as 1/2 m v^2, where m and v are the mass and velocity of the particle.
There is a subtle difference between the quantity calculated by this compute and the kinetic energy calculated by the ke or etotal keyword used in thermodynamic output, as specified by the thermo_style command. For this compute, kinetic energy is "translational" kinetic energy, calculated by the simple formula above. For thermodynamic output, the ke keyword infers kinetic energy from the temperature of the system with 1/2 Kb T of energy for each degree of freedom. For the default temperature computation via the compute temp command, these are the same. But different computes that calculate temperature can subtract out different non-thermal components of velocity and/or include different degrees of freedom (translational, rotational, etc).
This compute calculates a global scalar (the KE). This value can be used by any command that uses a global scalar value from a compute as input. See Section_howto 15 for an overview of LAMMPS output options.
The scalar value calculated by this compute is "extensive". The scalar value will be in energy units.